PHOENIX — Your car tells you it’s 105 degrees outside, but then you tune in to 12News that night and find out the actual high temp for the day was 99. Why is there such a difference?
The simple answer is that they are measured by two separate devices. A thermometer measures one while the other is measured by a thermistor.
VERSIÓN EN ESPAÑOL: La temperatura se mide de manera diferente según su automóvil le explicamos por qué
A calibrated instrument, a mercury thermometer, determines our official high temperature for the day.
It’s placed a little more than 6 feet above a grass surface to get an accurate reading of the air minus any added influence from a body of water, concrete or metal. (All of which will influence the air temperature. Think of it like being in the shade versus laying on a concrete block in direct sun.)
The liquid mercury expands inside the gauge, rising to a certain value from the heat. As the heat in the air cools, the fluid drops.
Your car displays a temperature measured by a thermistor. This is a device that measures heat radiated from surfaces around the sensor.
The most common location for a thermistor is behind the grill at the front of the vehicle. This means the sensor is measuring heat re-radiated from the surface of the road, grill and other contributing surroundings.
To drive this home for you, imagine walking on a sun-baked asphalt surface versus a grass surface. The asphalt is much warmer because darker surfaces absorb more solar radiation than grassy or shaded areas.
Thermistors are closer to giving an accurate value on cloudy days when the heating from the sun is lost. Along with nighttime temps for the same reason.
Temperatures displayed on signs from a business are also measured with a thermistor. These signs are often based in a parking lot surrounded by black tops or metal roofs. As a result, numbers are usually 10-plus degrees higher than the actual air temperature.
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